Signal transmission system



Y maybe of various kinds.

Patented Oct. 11, 1932 UNITED- scmTf1-:5s.` PATENT orifice i JGSEF SEDLMAYER, 0F BERLIN-CHARLOTTENBUBG; GERMANY, ASSIGNORTO SIEMENS AN D ELSKE AKTIENGESELLSCHAFT, OF SIEMENSSTADT, NEAR BERLIN, GERMANY,

L: GERMAN COMPANY SIGNAL TRANSMISSION SYSTEM Applicationled NovemberA 10,-y 1931, Serial N'o. 574,119,1and in Germany November 6, 1930.

depend upon the uniform regulation ofthe Whole signalband, i. e., the same degree'of control is appliedto all the frequencies'. A disadvantage of' these methods is that they do not correct'for selective fading phenomena. which cause variations in the relative transmission of variousfrequencies in the signal band. Y

In accordance Witha-feature of the present invention these disadvantages are reduced by r the. factthat the frequency range to be controlledissub-divided and each ofthe component bands is controlled perse.' Moreover, in order to obtain a particularly good control, both the transmitted-signals and pilot tones are employed't'o effect the control.

The causes of selective fading phenomena Insofar as it is a question. of interference phenomena, these may be'` eliminated within certain limits by making the transmission paths as clear 'as possible.-k In accordance With the present invention fading phenomena, irrespective of their causes, may be rendered harmless byselective control. Theoretically, the 'best control fis' obtained loydividing. the frequency band-t0 be transmitted into sub-bands, each as narrow as possible and providing `each of the sub-bands with its own control.` However, suchan extensive sub-division need not be made inv practice, for by dividing the signal into a small number of component bands,

c even two, considerable improvement is obtained.4A The sub-division should preferably be performed in such a Way that'each of the however.

component bands includes a substantial portion of the signal energy. l

There are three possibilities of effecting the control either in the case Where component bands are used or in the case Where the signal band is not subdivided. It has been proposed to allot to the frequency band to'be controlled a simultaneously transmitted pilot tone or control frequency which is used to control the degree of amplification or gain of the receiver. Another method is to employ the transmitted signal currents themselves for the control of the receiver.

There are disadvantages in both methods,

The first method has the disadvantage that the signal frequencies are controlled accordingto-the attenuation` of the control frequency and not according to their own attenuation.l Thus, When the control frequency and signal frequencies vundergo fading of different intensity (selective fading), this method of control will result in such an adjustment of the amplitudeof the signal as to correct'for the fading of the control frequency and may result in an increased distortion rather than a correction.

Thesecond method is applicable very Well to all the cases in which the signals are of constant amplitude, vvithoutpauses in transmission. On the other hand it is ineicient in speech transmission, for the reason that the fluctuationsy of amplitude of the signal are. considerable and must not be leveled. n In order to prevent such a leveling as would distort the character of the speech, there mustbe employed controlling devices of large time constants. These control circuits, conse# quently, cannot follow rapid fadings. This method has a further disadvantage that inthe speech pausesvthe atmospheric disturbances are raised to speech intensity. Moreover, since in this methodof control the vtransmission of the speech path is dependent upon the speech amplitude, theV variations therein cause a tendency to howl. (See E. N. T. 1928,`

volume 11, H. F. Mayer ber automatische amplitudenbegrenzer) In `accordance With the presentinvention e the advantagesof both methods areY combined and their disadvantages eliminated.

This is accomplished by eecting the gain control in accordance with both the signal currents themselves and a special control current having at the transmitter such an amplitude that when rectified along with the signal currents there results a rectified current of a constant value. In the receiving apparatus the degree of amplification or gain is controlled in response to the current resulting from the rectified received signals and control current. After the control current is filtered out of the resulting wave, there remains only the signal currents. Preferably, the average amplitude of the control frequency is chosen small with respect to the signal frequency amplitudes, so that the ampli-tude of the controlled wave is primarily dependent upon the latter. During the signal pauses the control frequency alone is present, so that the degree of amplification of the repeater is adjusted to a moderate level and interference noises are not introduced,

If several control tones are required, then it is preferable to choose their frequencies according to integral or approximately integral proportions, in order to avoid the formation of combination tones within the voice band. Thus, for example, a fundamental tone and overtones or the overtones alone are employed. These may be produced by generators or oscillators of well linown type.

The frequencies of the control tones are preferably so chosen that they are located on the edges of the frequency bands to be regulated. They can then be very easily filtered out without signal frequencies being suppressed with them. In the case where the signal band is divided into two component bands it is advantageous to select the frequency of the control tone so that it is located in the middle of the total band, i. e., between the two component bands, since in this way the limits of lthe component bands may be so chosen that harmonics of the lower control frequency come outside the signal band. This latter fact, that is, that the overtones of the control frequencies fall outside the voice band, is advantageous in every case. In the arrangement of the control frequency in the middle of the total band, the only disadvantage is that the signal frequencies in the immediate vicinity of the control frequencies may also be filtered out simultaneously therewith. With filters of sufliciently small band width, however, the harmful influence to intelligibility is rendered so small that it is outweighed by the advantages.

This method of employing signal currents and control currents jointly for the gain centrol, may not only be employed for the case of sub-division of the signal band as described herein, but also in the case where the band to be controlled is not sub-divided.l

The invention may be more readily understood by reference to the following detailed description in connection with the drawing in which:

Fig. l is a graph showing a survey of the frequency distribution of human speech; and

Figs 2 andv 3v show vdiagrammatically a radio telephone system embodying thefinvention, Fig. 2 showing the transmitter and Fig. 3, the receiver.

Fig. l shows asurvey of the 'frequencies contained in various sounds of human speech. The full lines indicate which frequencies are primarily contained in the sound concerned. The frequencies over which the dotted lines extend are also contained to a small extent in the sounds concerned,but without being of importance for the specific character. By means of such a diagram the considerations which are decisive for the frequency subdivision may be ascertained, The intelligibility losses following therefrom are decisive for the number land position ofthe component portions. In most cases it will be of advantage to make a sub-divisionV in two ranges so that each embraces a main forming range. y

Figs. 2 and 3 show diagraminatically a radio telephone system embodying the invention. The elements of the system are all circuits of well known types and consequently are shown only in block diagram form.

Let us first refer to Fig. Q which shows the transmitter. A telephone subset 10 including a telephone transmitter is connected through a bridge transformer ll to the lines l2 and 18 leading respectively to the radio transmitter and to a radio receiver (not shown). The audio frequency currents pass from the transmitter of the subset l() through the transformer 1l and line l2 to the junction point lll where the path is divided into three branches l5, 2O and 30. transmission path. The two branch lines 2O and 30 provide for the control of the pilot currents. The voice band is divided into component or sub-bands in these paths 2O and 30 and separate control or pilot tones supplied for each sub-band. p

Branch 20 contains a band-pass filter 21', which for example, passes' frequencies of 3004500 cycles. The output of this filter is supplied to an amplifier-rectifier 2,2 of the well known type. Similarly, branch 3() contains a band-pass filter 3l having la range of free transmission of G-2500 cycles and an amplifier-rectifier 32.

The transmission path 15 itself transmits the whole voice frequency range (SOO-2500 cycles) so that the signal currents are transmitted to the radio transmitter proper 16 over this path, which contains only' a delay circuit l?. This circuit is designed to compensate for the phase shift in the control paths 2O and 30 so that the pilot-tones introduced will bear the proper phase or timorelation to the signal currents. l

Branch l5 is thev 'v` The partial frequency bands flowing through the control paths effect the adjustment of the control currents (pilot'tones) by means of the amplitude controllers 23 and 33. rihese control currents are generated in an oscillator 13 which is of the harmonic generation type producing harmonic waves of 1500 and 3000 cycles.v The filters or tuned circuits 33 and 23 are designed to select the respective pilot tones of 3000 and y1500 cycles. The filter 33 may have cut-off frequencies of 2700 and 3300 cycles and selects the 3000 cycle tone which is allotted to the frequency band of 1500-2500 cycles. Similarly, the filter may be a low pass filter having a. cutoff frequency of 1300 cycles and operates to select the 1500 cycle tone which is allotted to the 300-1500 cycle sub-band.

n The 1500 and 3000 cycle pilot tones are supplied to the amplitude control circuits 23 and 33 respectively. rfhese circuits are similar in action and are designed in accordance with the well-known principles of operation of gain control amplifiers. For example, circuit 23 operates to control the amplitude of the 1500 .cycle pilot tone in accordance with the amplitude of the rectified current output of the amplifier-rectifier 22 which represents at 'any instant the average amplitude of the voice currents in the G- 1500 cycle sub-band. rlhe circuit is designed to effect such a control that when the resulting 1500 cycle pilot tone is rectified along with the 30G-1500 cycle sub-band the resultant rectified current will be constant in amplitude.

rlhe amplitude control circuit 33 is similarly responsive to the 1500-2500 cycle subband to produce such a control of the 3000k cycle pilot tone that when it is rectied 'along with the 1500-3000 cycle sub-band there results a constant current.

The control tones regulated in this manner by the component frequency bands allotted thereto are retransmitted together with the signal currents to the radio transmitter 16 where they modulate the carrier wave andare radiated from the antenna.

As described above the delay circuit 17 performs the function of equalizing the time of transmission in the transmission path 15 with that of the two control paths 20 and 30 so that f there will be a 'proper phase relation hetuf'een the signal currents and the introduced pilot tones.

Fig. 3 shows the receiving system. The radio waves received in the antenna are amplified and detected inthe radio receiver e0. The resultant signal and pilot currents leaving the receiver are divided by means of the band-pass filters 24 and 34 into two frequency bands. The filter 24 has a. pass-range of 300-1500 cycles and the filter 34, a passrange of 1500-3000 cycles. The currents of these twofrequency bands are then controllably amplified in the controlled amplifiers 25 and 35, and transmitted through the uni'- lateral transmission circuits 26 and 36, which by preventing 'back ,couplings increase the 'stabilityof the control circuits. he outputs l ofthe circuits 26 and 36 are each divided into two paths, one leading to the inputs to the respective amplifier-rectiers 27 and 37. The currents are rectified therein and supplied to the control amplifiers-25 and 35 respectively 75 operating to so control the gain thereof as to compensate for the effects of fading.

The other paths lead to the band elimination 1ilters28 and 38 which are designed to filter out the 1500 and 3000 cycle contro-l 3 tones, respectively. These filters are designed to be sharply selective to the control tones and to pass all of the signal frequencies. The remaining signal currents are thenrecombined and transmitted through a bridge transformer 41 to the subset 42 which includes a telephone receiver. f

The control amplifiers 25 and 35 are'designed in accordance with the well known principles of operation of'gain control amplifiers so that they operate to always maintain the output of the respective amplifierrectifiers 27 and 37 constant in amplitude.

The arrangement according to this invention may be combined with known devices for secrecy of the sub-divided signal band type. `In this way certain of the circuit elements may be used to perform two functions. Prefl erably,l particularly. in the transmission of speech, the combination is made with a secrecy system in which the signal band is not only sub-divided but the sub-bands are also displaced in the frequency spectrum. The filtering means necessary for the sub-division may be so dimensioned that they can also be employed for the control arrangement so that thusa substantial reduction of circuit elements and consequently'ef cost results In order ,toireduce the disturbing influence of non-linear distortions, such as may be produced lby selective fading phenomena, it is advisable to transform the signal frequencies for the transmission to a frequency range for which the differential tones are below Y and the overtones above the band. The rela- H5 tive position `of the control frequencies to the voice'f-requencies can be maintained. Thus,

for enamplainstead of the band 300-3000 cycles, the band 400G-6700 cycles may be employed. This band is obtained by hcterodyning with .3700 and 7000 cycle waves, respectively. It will be seen that the highest `differential'tone (2700 cycles) is below this band and the lowest overtone (8000 cycles) above this band. At the receiving station the shifted band is restored to its proper place in the frequency spectrum in a. corresponding manner. Amplitude control and complete or substantial filtering out of the control frequencies are preferably performed in the shifted frequency band, i. e., before the reconversion.

Also, in cases in Which there is no such frequency transformation at the transmitting station, it is advisable at the receiving station to first shift the signal frequency to a. suitable place in the frequency spectrum, by means of heterodyne reception. The control amplifiers and pilot tone filters are then operated in this frequency range and after control and filtering the resultant band is shifted back to its original position in the frequency spectrum.

The improvements obtained by the arrangement according to this modification may be so considerable that thereby the use of suppressors is rendered unnecessary. For this reason the extra cost is not material, particularly when the improvement in intelligibility is taken into consideration. Moreover, it is possible to employ one or both control frequencies for telegraphy.

What is claimed is:

1. In a radio signaling system, thermethod of reducing the effects of fading which comprises allotting to each of a plurality of portions of the signal band a control tone of signal frequency located substantially at a limit of the corresponding portion of the signal band and having an amplituderwhich bears a definite relation to the amplitude of the Waves in the respective portions of the signal band, separating the received Waves into sub-bands, each corresponding to one 0f said kportions of the signalband, separately amplifying each sub-band and controlling the amplification of each sub-band in accordance with both the amplitude of the respective control 'tone and the amplitude of the current in said sub-band.

2. In a radio signaling system, the method of reducing the effects of fading which comprises allotting to each of a plurality of portions of the signal band a control tone the amplitude of Which bears a definite relation to the amplitude of the Waves in the respective portions of the signal band, said control tone being harmonically related, separating the received Waves into sub-bands corresponding to one of said portions of the signal band, separately amplifying the sub-bands, and controlling the amplification of each subband in accordance With both the amplitude .of the respective control'tone and the amplitude of the current in said sub-bands.

In Witness whereof, I hereunto subscribe my name this 8th day of October, 1931.

JOSEF SEDLMAYER. 

